Resistance to thyroid hormone does not abrogate the transient thyrotoxicosis associated with gestation: report of a case.

We report the occurrence of transient thyrotoxicosis during pregnancy in a subject with resistance to thyroid hormone. Before pregnancy, the subject was euthyroid, with normal serum TSH and elevated levels of free T(3) and free T(4) caused by a mutation in the TRbeta gene (R243Q). Beginning at the fourth week of gestation serum levels of free T(3) and T(4) increased in parallel with an increase in hCG. At 6-7 wk gestation she manifested hypermetabolic features, with mild nausea and vomiting. Peak levels of serum hCG and thyroid hormone concentrations were attained at 12 wk gestation, when serum TSH was fully suppressed. In the following weeks of gestation, thyroid hormone levels declined, with amelioration of the symptoms. A baby boy also affected with resistance to thyroid hormone harboring the same TRbeta gene mutation was born by normal vaginal delivery.